Frontiers in Human Neuroscience (Apr 2016)
Event related potentials reveal early phonological and orthographic processing of single letters in letter-detection and letter-rhyme paradigms
Abstract
Introduction: When and where phonological processing occurs in the brain is still under some debate. Most paired-rhyme and phonological priming studies used word stimuli, which involve complex neural networks for word recognition and semantics. This study investigates early (300ms) orthographic and phonological processing of letters.Methods: Eighteen participants aged 20-35 engaged in three two-forced choice experiments, one letter-detection (LetterID) and two letter-rhyme (Paired-Rhyme and Letter-Rhyme) tasks. From the EEG recordings, ERP differences within and across task stimuli were found. We also calculated the global field power (GFP) for each participant. Accuracies and reaction times were also measured from their button presses for each task. Results: Behavioural: Reaction times were 18ms faster to letter than pseudoletter stimuli, and 27ms faster to rhyme than nonrhyme stimuli. ERP/GFP: In the LetterID task, grand-mean EPs showed typical P1, N1, P2, and P3 waveform morphologies to letter and pseudoletter stimuli, with GFPs to pseudoletters being greater than letters from 160-600ms. Across both rhyme tasks, there were greater negativities for nonrhyme than for rhyme stimuli at 145ms and 426ms. The P2 effect for rhyme stimuli was smaller than letter stimuli when compared across tasks.Conclusion: Differences in early processing of letters versus pseudoletters between 130-190ms suggest that letters are processed earlier and perhaps faster in the brain than pseudoletters. The P2 effect between letter and rhyme stimuli likely reflect sublexical phonological processing. Together, findings from our study fill in evidence for the temporal dynamics of orthographic and phonological processing of single letters.
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